1. The Field of the Invention
The present invention is directed to extruders. More particularly, the present invention is directed to the wear surfaces of extruder screws.
2. Description of the Related Art
Twin screw extruders typically comprise a barrel having two overlapping bores of equal diameter and mounting hardware for accepting and receiving two extruder screws, which rotate in opposite directions with the threads meshing. The barrel is made from a hard steel. The interior of the bores is typically processed to provide a hard surface. The screws are made of a high grade steel.
The screws must seat tightly against the inner walls of the bores if the extruder is to force the extrusion material through the barrel efficiently. If the screws do not fit tightly into the bores, much of the energy put into the counter-rotating screws churns and mixes the extrusion material and forces it between the sidewall of the bores and the screws. This energy does not move the extrusion material forward. When the gap between the screws and the bores becomes great enough, very little, if any, extrusion material is extruded.
Accordingly, manufacturers generally provide screws that fit tightly into the bores of the barrel, but have contact points of wear material, or bearing surface, that is softer than the barrel, thus permitting a snug fit between the screws and the bores, while insuring that the screws will wear faster than the bore, which is more expensive than the screws.
This problem has been addressed by providing extruder screws having flattened crests on the flights, with a groove cut into the crest, leaving an edge at each side of the thread crest. The outer diameter of these edges is less than the diameter of the bore. A weld bead is laid down on these edges, which are built up to a greater diameter than the bore. Then an even softer material, such as molybdenum, a silvery hard metallic transition element, is molted and sprayed into the groove by an automatic machine until it is built up as high as the weld beads. Next, the wear surfaces, or bearing surfaces, of the extruder screws are machined to the desired diameter, i.e., the diameter of the bores of the barrel, and completed to the desired RMS finish.
Extrusion screws, or screw augers, are frequently 15 or 20 feet (5.7 m-6 m) long and have a thread of some type, whose characteristics may vary from one section of the screw to another, that runs nearly the length of the entire screw.
The bond between the screw and the molybdenum, or other bearing surface, is a mechanical bond. The molybdenum and steel screw are not heated to a high enough temperature to cause fusion bonding. If significant fusion bonding did occur, rebuilding the screw would be much more difficult. In use, the mechanical bond between the bearing surface and the groove in the screw can break. The force of the extrusion materials being forced through the barrel frequently rips off long sections of the bearing surface from the extrusion screws. In this case, the screw no longer fits tightly into the bores and the extruder does not work well, if at all. The extruder must be shut down, the screws removed from it, and replaced with a different set of screws. This process puts the extruder out of service for five to ten hours. Many shops have only one extruder and operate it 24 hours a day. The failed screws must be rebuilt. Such shutdowns are obviously costly. Moreover, they occur prior to the time that normal wear would require a shutdown.
Accordingly, there is a need for an extruder having screws with a bearing surface that cannot be stripped out of the screw in significant lengths if a portion of the bearing surface breaks free of the screw.